HO INTRODUCTION TO 



scutellum sometimes extends over the anterior edg j 

 of the metathorax ; in some instances it entirely corj 

 ceals its surface. An impressed line sometimes sepa 

 rates an anterior portion from the rest, and in such 

 case this is called the Postscutellum. As the intern* 

 cavity of the mesothorax is separated from that <i 

 the metathorax, by a partition denominated th 

 Mesophragma, so the latter is separated from th 

 abdomen by a similar parietal wall, called the Meta 

 phragma, a small space only being left for the passag 

 of the necessary organs. The ventral plate of th 

 metathorax is the metasternum, a portion whic 

 most commonly assumes a shape similar to that c 

 its counterpart above, but it varies greatly in som 

 of the orders. On each side of the thoracic segmen 

 now under consideration, are found two horny piece 

 which Kirby and Spence have named plura ani 

 periplura. In the latter, the halteres or poisers o 

 Dipterous insects are situated. 



We have now to consider the organs attached to th< 

 thorax, by which motion is effected either on the lam 

 or in the air, viz. the legs and wings. The former ar 

 never wan ting, and their number is invariably the same 

 the latter are often absent, and when present, thei 

 number varies from two to four. Such kinds as posses 

 four wings, are called Tetr apterous insects; those wit! 

 two, Dipterous ; and when altogether wanting, th> 

 species are said to be Apterous. The point at whic] 

 the wings are inserted, has been already indicated 

 they articulate to the thorax by means of numerou 

 small pieces. Viewed relatively to each other, tin 



